This class is the base for various encoders and decoders, which are used during transport of the message. These packages, and all which are derived, are invoked by the message's Mail::Message::decoded() and Mail::Message::encode() methods:

Fatal error: the specific package (or one of its superclasses) does not implement this method where it should. This message means that some other related classes do implement this method however the class at hand does not. Probably you should investigate this and probably inform the author of the package.